In recent years, the importance of approach to energy issues has grown out of concern over global warming due to greenhouse gases. It is a pressing need, in particular, to break away from dependence on fossil fuels. Under such circumstances, electric generation by the photoelectric conversion of solar light energy, i.e., a solar cell is a key to solving the above-described problem.
Currently, most solar cells use silicon (Si) as the raw material thereof. On the other hand, a solar cell using gallium arsenide (GaAs) which is a compound semiconductor has been put into practical use in applications, such as mounting on an artificial satellite, requiring high photoelectric conversion efficiency, for reasons of matching with a solar light spectrum.
Next, the configuration of such a solar cell will be described with reference to FIG. 12. FIG. 12 is a block diagram illustrating, by way of example, the configuration of a conventional pn junction-type semiconductor solar cell 100. The solar cell 100 has a known configuration comprising an n-type semiconductor layer 102 and a p-type semiconductor layer 104 formed on the n-type semiconductor layer 102. These layers are formed as a unit cell by using known semiconductor manufacturing techniques and the like. However, solar cells can use only the solar light having a wavelength range corresponding to a forbidden band width (bandgap energy) that the material of the solar cells has. Accordingly, the solar cell composed of the unit cell is limited in the photoelectric conversion efficiency thereof.
Hence, there has been proposed a structure of a tandem solar cell intended to widen the available range of solar light spectra and improve photoelectric conversion efficiency. This tandem structure is composed of a plurality of unit cell structures similar or identical to the solar cell 100 illustrated in FIG. 12, where the respective unit cells have different forbidden band widths and are stacked along an entering direction of light from a light-receiving plane, in the order of the widest to narrowest forbidden band widths. For example, the tandem solar cell described in U.S. Pat. No. 7,217,882 (hereinafter referred to as Patent Literature 1) has a forbidden band width corresponding to 3.4 eV to 0.7 eV and is, therefore, compatible with a wide wavelength range of solar light spectra.
As another technology, there has been proposed a solar cell comprising a quantum well structure. For example, the solar cell described in Japanese Unexamined Patent Application No. JP-A-2002-141531 (hereinafter referred to as Patent Literature 2) has a configuration including a plurality of convex portions on a quantum well surface.
The current conservation condition that respective cells constituting a tandem structure generate the same excited current is required to be satisfied, in order to realize high photoelectric conversion efficiency in a tandem solar cell. In other words, an absorption edge wavelength, i.e., a forbidden band width in each cell, has to be precisely controlled, in order to equalize photoexcited currents generated in the respective cells.
The material composing the tandem solar cell described in Patent Literature 1 is a mixed crystal of indium gallium nitride (InGaN). InGaN is strong in thermodynamical immiscibility, and therefore, an inhomogeneous microscale mixed crystal composition distribution takes place. This means that the forbidden band width of InGaN is not uniquely determined. Accordingly, the inventors have clarified that it is difficult to precisely control the absorption edge wavelength of each cell when a tandem solar cell is formed of InGaN.
On the other hand, the material composing the solar cell described in Patent Literature 2 is a GaAs-based material. With this material system, it is possible to use only part of a solar spectrum. Thus, photoelectric conversion efficiency as high as that of a tandem solar cell cannot be expected from the solar cell.
In addition, the solar cell described in this Patent Literature 2 has a configuration including a plurality of convex portions on a surface of a quantum well layer. This means that an absorption edge wavelength is varied by the convex portions of the quantum well layer. Accordingly, the inventors have clarified that it is difficult to precisely control the absorption edge wavelength of the solar cell described in this Patent Literature 2, and therefore, the solar cell is unsuitable for forming a tandem structure.
In addition, if the potential barrier of the quantum well structure is high and carriers localize deep in the well layer, it is no longer possible to take out the carriers from the quantum well layer as a current. Also from this point of view, it is a critical issue to precisely control the forbidden band width of a material composing the solar cell.
The present invention has been accomplished in view of such circumstances as described above. Accordingly, an object of the present invention is to provide a solar cell which facilitates precise control of the absorption edge wavelength of each cell in a tandem solar cell and has high photoelectric conversion efficiency, and a method for manufacturing the solar cell.